Currently, most of the dental canal drilling instruments have a cutting part called a cutting section with a conical envelope and including one or more cutting edges wound up into a helix along this cutting part.
This helical shape is indispensable for removal of debris from teeth around the outside of the root. During use, the conical part may undergo a coating phenomenon when the instrument is introduced into the root canal and when it is carried along in rotation. It may happen that it becomes bound in the root canal. For this reason, it is recommended to use such an instrument only with manual axial movement possibly with a slight movement in rotation alternating in one direction and in the reverse direction. Otherwise, the instrument may be blocked in the tooth and break. Breaking the instrument may cause serious consequences given that a dental canal is narrow and that access to it is difficult.
There are certain instruments for which the tendency towards binding has been partially eliminated. In particular, this has been obtained by greatly blunting the cutting angles. This type of instrument may be used with a rotating drive motor turning at low speed so as not to deform the natural path of the canal while enlarging it.
The fact that the cutting edges may be blunted presents a drawback for several reasons. On the one hand, the cutting work necessary to enlarge the dental canal is carried out with difficulty. On the other hand, the friction between the blunted edges and the walls of the canal causes significant working torque, which may also cause the instrument to break. In order to avoid this risk, it is necessary to use a whole range of instruments with different conicities, those having the greatest conicity being used first. In this case, the coating effect is eliminated and the restraints due to friction caused on the instrument remain less than the breaking limit. However, the work is only applied to a delimited part of the dental canal.
With most of the known motorized drive instruments the gain in time relative to the manual method is not certain and the risks of breaking remain high.
Various instruments of this type have been the object of patents. The U.S. Pat. No. 6,074,209 precisely describes one of these instruments in which the edges are rounded. It includes zones with narrow cross section which alternate with zones of larger cross section. The zones of narrow cross section are obtained by grinding the part in predetermined places of a blank in which the section is initially constant if the instrument is cylindrical, or regularly decreasing if the instrument is generally conical in shape which causes passages between the zones with narrow cross section and zones with larger cross section to sharp transverse edges which risk catching on the walls of the canal and blocking the instrument.
The international application published as WO 00/59399 describes an instrument of this type made by twisting an elongated piece with a general conical shape and in which the cross section is rhomboidal in shape. It is also the case with the instrument described in U.S. Pat. No. 4,260,379. The rhombus has a large diagonal and a small diagonal, the instrument is made twisting a piece in which the section is a rhomboid and includes alternating zones of reduced cross section and larger cross section corresponding respectively to the small and large diagonal of the rhombus. The zones with different section are not moved along the instrument but they are angularly moved by 90 degrees. It is noted that this geometry does not allow avoidance of the binding phenomenon of the instrument and that certain restraints exerted on this instrument could cause its breaking.